Radiation-sensitive mesh defect inspection system

ABSTRACT

A mesh defect image inspection system incorporating a flying spot scanner tube together with an anticorrelator circuit utilizing two fixed delay lines of different lengths and matching the difference in delay time to a multiple of the time required to move the flying spot between two adjacent holes in the mesh member. The inputs of the delay lines in the anticorrelation circuit are in parallel and outputs in series opposition. A display means is provided, for example a video monitor tube. If a defect results in a mismatch of signals, a bright spot will be displayed and the detect may then be pinpointed.

United States Patent [72] Inventor Rudolf C. Hergenrother 3,099,748 7/1963 Weiss 250/236 X West Newton, Mass. 3,158,748 1 1/1964 Lacak et al. 250/219 [21] Appl. No. 828,612 2,208,447 7/1940 Berry 26/515 [22] Filed May 28, 1969 2,680,200 6/1954 Hercock 250/219 [45] Patented Nov. 16, 1971 3,410,643 11/1968 .lorgenson 250/219 Asslgnee Raylheon Company Primary Examiner-Walter Stolwein Lexington Mass AllrneysHar0Id A. Murphy and Joseph D. Pannone [54] RADIATION-SENSITIVE MESH DEFECT INSPECTION SYSTEM 16 1 1 13 D i Fl v C 8 ms raw ng gs ABSTRACT: A mesh defect image inspection system Incor- [52] US. Cl 356/237, i a fl i Spot scanner tube together with an anticop 250/219 250/217 relator circuit utilizing two fixed delay lines of different [51] Int. Cl G01n 21/32 lengths and matching the difference in delay time to a multiple [50] Fleld of Search. 250/219; of the time required to move the flying Spot between two 26/51- 0? 250/203 236i 217 CR; jacent holes in the mesh member. The inputs of the delay lines 356/199 238 in the anticorrelation circuit are in parallel and outputs in series opposition. A display means is provided, for example a [56] References Cited video monitor tube. lfa defect results in a mismatch of signals, UNITED STATES PATENTS a bright spot will be displayed and the detect may then be pin- 2,878,712 3/1959 Blackstone 250/220 X pointed.

DELAY 52 44 45 LINE 50 I YOKE SAWTOOTH BLANKING BLANKING DRIVER GENERATOR CIRCUIT CIRCUIT .Y I y /s L.V. 32 CO CONTROL GRID L.V. ANODE 34 ANODE J GRID l2- /4 BEAM CENTERING 36 22 FOCUS con. DEFLECTION YOKE 40 42 26-" H.V. 28 H.V. ANODE ANODE WRITING REA ruse l 9&

OBJECTIVE LENS PHOTO SENSITIVE i l MESH UNDER FILM L INSPECTION L I 64 #CONDENSER LENS 6' 1 MULTIPLIER PHOTOTUBE AMPLIFIER FILTER 72 L ANTI CORRELATION PAIENTEDIIIII 16 Ian ANTI CORRELATION SHEET 1 0F 5 DELAY LINE 44 46 so I 1 YOKE SAWTOOTH BLANKING BLANKING DRIvER GENERATOR CIRCUIT CIRCUIT 48 II I 30 2 CONTROL /8- CONTROL 0 Eo 7 GRID L.V. O--J- GRID AN D ANODE /2 (BEAM CENTERING 36 22 FOCUS con. 3 DEFLECTION YOKE 20 26" H.\/. 28 H.V. ANODE ANODE II I WRITING READING TUBE I TUBE Y |-a6 l 3 l/ 0BJEcTIvE- LENS PHOJOE l sENsI Iv I I 6 MESH uNDER FILM Q 1 I j INSPECTION [-0 62/ I 1 CONDENSER LENS MuIJIPLIER PHOTOTUBE FILTER l/V VE N 7 0R RUDOLF CZ HERGENROTHEI? ATTORNEY 'PATENTEUN V 16197! 3520.630

SHEET 2 [IF 5 LTERNATE LINE SCAN DIRECTION FLYING SPOT FIG 2 L|NE*| EFFECT OF EFFECT OF 2 APLUGGED AN ENLARGED HOLE HOLE A A A A A A g V \l \l \1 1 \I 5 T -TIME DELAY To 3 B A A A /A A A 4 ANTI-CORRELATION FILTER 2 C I A W *TO* TlME-- VIDEOINPUT FROM MULTIPLIER es ACOUSTIC Tfifiliflr'fm I x m6 2 TERMINATION if 1/ 1 7 4,- J L- //0 l SONIC //2 WDEO WAVE TRAVEL 7 VIDEO OUTPUT OUTPUT 'vwvv E I 1 ANTI- CORRELATION Fla 4 FILTER OUTPUT INVENTOR RUDOLF C HERGENROWER an? I 5' W A ronwn PATENTEDIIIIII I6 I97! 3,620,630

SHEET 3 OF 5 DELAY LINEQN FIG 5 INPUT FROM 1} ANTI- CORRELATION MULTggLIER j q, OUTPUT VIDEO OUTPUT FOR MESH IMAGE VIEWING ON MONITOR COAXIAL CABLE C IV INPUT CHARACTERISTIC k 0F 0 0 IMPEDANCE 2 20 I34 vIDEo ANTI-CORRELATION OUTPUT FOR MESH DEFECT VIEWING 0N MONITOR AND FOR RECORDING vIDEo OUTPUT FOR MESH IMAGE vIEwING ON MONITOR k /44 VIDEO CHARACTERISTIC I/ INPUT 'MPEDANCEW I CHARACT RISTIC rSHORT 0F 0 IMPEDAN E 20 ClRCUlT I40 vIDEo ANTI-OORRELATIONMZ OUTPUT FOR MESH DEFECT FIE 7 VIEWING 0N MONITOR AND FOR RECORDING MONITOR CR TUBE 2 VIDEO R120) coAxIAL LINE vIDED EHARAcTERIsTIc o CHACTERISTIC INPUT IMPEDANCE 2 IMPEDANCE 0P2 R=ZO l/VVE/VTOR I50 RUDOLF 6./-IER6ROTHER 8 {SW/Ten $1 ATTORNEY [START OF SCAN SQUARE OPTICALLY ACTIVE AREA END OFSCAN STARTOFSCAN [MESH sQuARE OPTICALLY T I I ACTIVE AREA I l I T I i I 1 I i I l l I I 4 Y T 1 I I M6. m I i 1 i I I 1 L I I I l f L J J L l END OF SCAN 2L 35 L L, TRANSPARENT PLATFORM MESH= LXL PHOTOSENSITIVE FILM (L X L) OPTICALLY ACTIVE AREA A X A) IN I/[ N TOR RUDOL F C. HERGE/VROTHER sn g 5,8 4'

W 2 nzron/sr F76 Ila PAIENTEIIIIIII I 6 l9?! 3 620.630

SHEET 5 0F 5 SLOW-SCAN FRAME SLIDES oibgf fl L /GRO0VES OR RAILS l SLOW-SCAN I i DRIVE MOTOR I -20/ I I 206 F I g I I SLOW-SCAN SLOW-SCAN I LIMIT SWITCH LIMIT SWITCH I i I zoz ifyz-L 5 :j-s 204 LEAD sc w ZOPTICALLY ACTIVE AREA Log F/G llb RAPID-SCAN DRIVE MOTOR RAPID-SCAN LIMIT SWITCH 2/0 20a /LEAD SCREW I 1 I g i 207 RAPID-SCAN FRAME SLIDES RAPID-SCAN 20/ ON TRAVEL =i'ai sLow- SCAN FRAME WITH snoovss OR I RAILS i -oPTIcAL ACTIVE #T AREAS 2/2 RAPID-SCAN LIMIT SWITCH N V5 11/ 7' 0i? RUDOL F 6'. HERGENROTHEI? A7 omvsy BACKGROUND OF THE INVENTION A need exists for automated systems designed for the inspection of perforated planar structures in the form of a twodimensional lattice, which is to say, having a repetitive pattern of holes of equal size and shape. For example, metal mesh. used in some types of electron imaging tubes has a thickness of one-fourth of a thousandth of an inch with square holes of one-half thousandth of an inch on a side arranged in a square pattern having one thousand holes per linear inch or one million holes per square inch. A system for inspecting such mesh for defects, such as plugged holes or enlarged holes, without resorting to the tedious process of using the human eye to scan the entire area of the mesh would be very desirable. Such a system should rapidly and automatically find and mark the location of defects which could then be processed as desired, i.e., magnified and examined by an operator.

The present invention accomplishes this automatic inspection by using a correlation operation. This becomes particularly simple and direct when the subject is a uniform periodic mesh. The mesh inspection system of the present invention scans a mesh and sees only the defects while being blind to the perfect parts of the mesh. In addition to making the defects visible to an observer, the system makes an exact map of defect location and character which can be directly used for quality control purposes.

The present invention uses, in part, a conventional flying spot scanner system. To meet the requirement of making the system blind to everything except imperfections, the video signals are processed in such a way as to cancel those signals produced by perfect parts of the mesh while allowing signals produced by imperfections in the mesh to pass through. By inserting an "anticorrelation filter in the video circuit link between the scanner tube and a monitor tube, the monitor shows only the defects in the mesh. Since the mesh, in general, will be much larger than the focused flying spot raster, it is necessary to sweep the mesh mechanically across the flying spot raster area in order to inspect the complete mesh area. This mechanical displacement of the mesh in its own plane is done at sweep speeds which are limited only by mechanical factors. The system maps the mesh defects on a sheet which can be matched with the'mesh itself to find the location of mesh defects and show the details of the mesh defect.

SUMMARY OF THE INVENTION The above objects and advantages of the present invention, as well as others are achieved by providing an image inspection system for inspecting perforated planar structures in the form of a two-dimensional lattice, said system comprising means for electrically scanning the structure being inspected; means for displaying the resulting scanning signal such that only defects in the structure are displayed; means coupled between the scanning means and the displaying means for processing the video signals from the scanning means so as to cancel those signals produced by perfect parts of the structure while allowing signals produced by defects in the structure to be viewed by the display means; means for sweeping .the structure relative to the scanning means to permit inspection of the entire area of the structure; and means for mapping defects in the structure which are displayed by the display means.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram of the image inspection system of the present invention;

FIG. 2 shows a mesh being swept by a focused flying spot;

FIG. 3 are plots of the voltage versus time for the sweeping action shown in FIG. 2;

FIG. 4 shows one version of the anticorrelation filter shown in FIG. I;

FIG. 5 shows another version of the anticorrelation filter shown in FIG. 1;

FIG.6 shows another version of the'anticorrelation filter shown in FIG. 1;

FIG. 7 shows another version of the anticorrelation filter shown in FIG. 1;

FIG. 8 shows another version of the anticorrelation filter shown in FIG. 1;

FIG. 9 illustrates one mode for scanning the mesh shown in FIG. 1;

FIG. 10 shows another mode for scanning the mesh shown in FIG. I; and

FIGS. 11 a, b and 0 show a mechanical scanning for moving the mesh and film shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a schematic diagram of the image inspection system 10 of the present invention. A flying spot scanner tube (reading tube) 12 and a monitor tube (writing tube) 14 are provided in parallel. The scanner tube 12 includes a cathode 16, a control grid 18, a low-voltage anode 20, a beam-centering coil 22, a focusing coil 24, a deflection yoke 26 and a highvoltage anode 28. Likewise the monitor tube 14 includes a cathode 30, a control grid 32, a low-voltage anode 34, a beamcentering coil 36, a focusing coil 38, a deflection yoke 40 and a high-voltage anode 42. The tubes 12 and 14 are identical and operated in parallel both in tube voltages and scanning coils. The deflection yokes 26 and 40 are both driven from a yoke driver 44. A sawtooth generator 46 applies. its output both to the yoke driver 44 and to a blanking circuit 48 and. to a blanking circuit 50 via a delayline 52. The blanking circuit 48 is connected to the control grid 18 of scanner tube 12 while the blanking circuit 50 is connected to the control grid 32 -of the monitor tube 14.

An objective lens 54 is used in conjunction with the scanner tube 12 to focus the scanned raster of the scanner tube I2 on the mesh 56 under inspection. An objective lens 58, which is the twin of the lens 54 used with the scanner tube I2, is used to focus the scanned raster of the monitor tube I4 on a photosensitive film 60. The photosensitive film 60 and the mesh 56 under inspection are attached to a common frame 62 which can be mechanically scanned in such a mannerthat the entire mesh. area is swept by the flying spot raster of the scanner tube 12. The monitor tube 14 thus scans in synchronism with the flying spot raster of the scanner tube 12. The brightness of the flying spot remains constant while the brightness of the spot on the monitor tube 14 will be suppressed as long as the flying spot is traversing a defect-free area of the mesh 56. The monitor 14 focused spot will become bright, to expose the photographic film 60, when a defect in the mesh 56 is being scanned. The developed photographic film 60 will present the desired exact map showing only defects in the mesh 56. The light in the flying spot of the scanning tube 12 passes through the mesh 56 into a condenser lens 64 which focuses the light on the cathode of a multiplier phototube 66. The output of the phototube 66 is applied to an anticorrelation filter 68 whose output is amplified in an amplifier 70 and then applied to the control grid 32 of the monitor tube 14 to cause the monitor tube 14 to display only defects in the mesh 56 onthe photographic film 60. The anticorrelation filter 68 may be bypassed by a switch 72 connected to a line 74 for permitting viewing of the mesh image on a display tube (not shown) when mechanical scanning is stopped and the.

monitor tube 14 is biased off.

In order to fully comprehend the operation of the system 10, various of the necessary elements will now be described in greater detail.

FIG. 2 shows a section of the square hole mesh 56 having uniformly spaced holes 84 across which a focused flying spot 86 is being swept. That portion of the light passing through the holes 84 in the mesh 56 will reach the multiplier phototube 66 to produce an electrical output signal. Successive horizontal scan lines 1-5 are shown. The fact that a scan line may lie between rows of holes is of no consequence. A video signal is obtained on a successive scan line to detect holes and defects. Misalignment between the line scan direction and the mesh hole alignment is kept to a minimum since misalignment causes a drift in the output electrical signal from successive holes as indicated by scan lines 2 and 4. Another line scan direction which might be chosen is on the diagonal to the hole squares 84. as shown by line 6. The period of the output signal fQL th1 's direction is greater by \/2 than for the first.

The focused spot has radial symmetry with a radial intensity distribution which is approximately gaussian. The resolution of this system is described in greater detail in Resolution of Flying Sp'ot Scanner Systems," IEEE SpeclrumAug. I967, page 77. The waveform of the electrical signal produced by the electron multiplier 66 is periodic, repeating the same pattern as the spot 86 moves from one hole 84 to the next. The periodic signal may have a complex waveform and have a high harmonic content, but this does not interfere with operation of the anticorrelation filter 68, so it is not necessary to compute the expected signal waveforms in detail.

Assume a square-shaped focused raster having a frame rate of 30 per second. Let a single line sweep of the raster cover N/a holes 84 of the mesh 56, where N is the number of lines in the raster. The time 1,, required for the focused spot 86 to move from one hole 84 to the next is given by As Mn is reduced, the output signal period frequency )1, diminishes twice as fast as N/a.

A lower limit on a is established by the video circuit bandwidth. Thus, a should be greater than unity in order to exceed the resolving power of the video system. It is desirable to choose a to be as small as permissible since this minimizes the number of mechanical sweep scans required to cover the entire mesh area. A value of a=2 is believed to be near an optimum value.

The operation of the anticorrelation filter 68 depends on the comparison of successive video signals coming from the photomultiplier 66. This is accomplished by using delay lines which can delay the video signal by one or more periods of the fundamental wave. In FIG. 3, the upper two traces show the electrical output signals of the two delay line branches as a function of time for one of the scan lines of FIG. 2. In this example, an arbitrary periodic signal shape is shown and the two delay lines difi'er in length by the distance required for the sonic wave to advance by one fundamental wave period. The lower curve shows the electrical output signal produced in the anticorrelator filter 68 by connecting the two delay line branch outputs in series opposition. It is seen that, where the mesh 56 is good, a zero output signal is produced, and, when a defect is scanned by the focused flying spot 86, this results in large anticorrelator filter output signals when the spot passes the boundaries of each defect. Plugged holes produce a defect signal as well as enlarged holes. The signal produced by a plugged hole is about half as large as that produced by an enlarged hole, and this characteristic may be used to distinguish electrically between the two defect types.

The anticorrelation filter 68 may take the form of a number of different types of delay lines. An ultrasonic delay line 108 is shown schematically in FIG. 4. The video input signal from the multiplier 66 operates an electromechanical transducer 102 which produces sound waves. The sound waves travel in two unequal length branches 104 and 106 of a sonic delay line 108 having acoustic terminations 110 and 112. At the outputs of the sonic delay line's branches, the acoustic waves are changed back to electrical video signals No. 1 and No. 2 by appropriate transducers 114 and 116 respectively. The difference in delay times between the two delay line branches 104 and 106 is made equal to an integral multiple of the repeat period t of the electrical signal. (It is advantageous, as will be shown later, to make this delay time equal to a single I, time unit rather than a multiple of these.) An ultrasonic delay line will be very short in length and will have about 20 db. attenuation for the fundamental frequencies required.

It is also possible to use loaded electromagnetic delay lines such as those made by Computer Devices Corp. of Courmak, NY. to delay the video signals directly. This would avoid the 20 db. attenuation produced by an acoustic delay line which would require added amplifiers. The electrical circuit which' could be used for an electromagnetic delay line 120 is shown in FIG. 5. This type of line, however, has considerable distortion, depending on its length, and cancellation of signals will be somewhat incomplete.

A smooth transmission line, such as a coaxial cable, will produce a minimum of distortion with very little attenuation and therefore would make an ideal delay line. It is possible to obtain complete cancellation by using a tenninated cable 130, as shown in FIG. 6. The anticorrelation output signal is taken from a tap on the input resistor 132 to compensate for the attenuation produced by the cable on the signal 3% the terminating resistor 134. The only problem with this kind of delay line is the small delay of 3% nanoseconds per meter, or roughly 1 nanosecond per foot of length, which requires a great length of cable.

A delay line element for reducing the required length of cable to a delay time of one-half the fundamental video wave period is shown in FIG. 7. The short circuit 142 at the end of the coaxial cable 144 will reflect a wave of opposite polarity back down the cable, giving a total time delay corresponding to twice the cable length, thus producing cancellation of the fundamental video frequency. Cancellation will not be complete because of the attenuation of the cable, which may amount to a few tenths of the db. to a half db. depending on the cable design. Cancellation of frequencies higher than the fundamental will be incomplete because of the dispersion characteristic of the resonant filter. Thus, its usefulness for an anticorrelation filter depends upon the video output from the repetitive video signal being low in higher harmonic components.

The delay line cable length can be reduced to one-fourth of the fundamental video wave period by using the delay line circuit in FIG. 8. This circuit has the advantage of simplified switching provided by switch 152 at the circuits termination to activate or deactivate the filter action.

The decay rate of the phosphor of the screens of tubes 12 and 14 have an effect on the operation of the system. The phosphor in the cathode ray tube fluorescent screens of tubes 12 and 14 produces light when struck by the electron beam. The brightness of this light does not instantaneously drop to zero when the electron beam is turned off or moved to another area on the screen but falls off at an exponential rate characteristic of the particular phosphor. This means that the spot of light 86 in the focused raster scanning the mesh 56 has a trailing edge which will also produce a signal in the photomultiplier tube 66. The efiect of this trail is to produce a distortion in the output signal. This distortion of the output signal does not alter the uniformity of the output signal waveform as long as the mesh is uniform; therefore, the anticorrelation filter 68 will cancel the signal produced from a uniform mesh area regardless of the nature of the distortion. Since distortion of the signal means increased higher harmonics, the use of resonant circuit elements as in FIGS. 7 and 8 may be handicapped. However, in areas of the mesh where there is a defect, the distortion signal will prevent complete cancellation of the signal by the trailing anticorrelation filter 68 until the signal produced by the trailing edge of the spot 86 has died out. Thus, the signal output from the anticorrelation filter 68 will show a trailing edge whose length depends on the phosphor decaying rate. Therefore, a phosphor should be used which has a high decay rate, such as P16 which decays to percent of its brightness in 0.12;. sec. after excitation ceases, which is about the time required to travel one spot width.

The photographic film 60 may consist of conventional types of photographic film for recording the mesh defect map. This requires shielding of the film from room light during exposure and requires the usual darkroom techniques of developing and drying the film before it can be handled.

It is possible to use a direct imaging film which requires no darkroom processing and can be handled immediately after exposure. Such a film is made by E. l. Dupont de Nemours Co. and identified as Instant Access Ultraviolet Imaging film UVlXF-l. This film has a maximum sensitivity in the region of 3,000 A. to 4,100 A., and its sensitivity drops to zero at 4,600 A. and it becomes deactivated or fixed by longer wavelengths. This film can be used with a monitor tube having a P16 phosphor (which produces light in the 3,400 A. to 4,800 A. region) by using an optical filter which cuts off radiation of wavelengths longer than 4,600 A.

Since the mesh 56 and the photographic film 60 are attached to a common frame 62, any linear motion of the frame 62 in its own plane will not distort the accuracy of the mapping operation. This means that the system will be insensitive to variations in the mechanical mesh scan rate even to the extreme of this being random, as long as speeds of several hundred inches per second are not exceeded. The parallel opera tion of the cathode ray tubes 12 and 14 insures the accurate duplication of the flying spot tube 12 scan by the monitor tube 14 scan. The parameter which must be controlled in the system is the uniformity of line scan speed of the flying spot raster and the alignment of the scan line with the mesh hole line pattern. The frame scan speed of the flying spot raster does not affect the performance of the system, as will be shown below. Thus, the frame scan does not need to be interlaced as in the conventional TV system and, indeed, the frame scan could be random without disturbing the system. It may be convenient, however, to use interlaced scanning because of the availability of equipment designed for TV systems use.

The mechanical scan velocity of the mesh 56 combines with the raster scan velocity of the flying spot 86 to determine the fundamental period of the video signal produced by the photomultiplier tube 66. The minimum effect on the video signal is obtained when the mechanical line scan motion of the mesh 56 is perpendicular to the line scan motion of the flying spot raster.

To consider a practical example, suppose the mesh 56 has 900 holes per inch and suppose we choose the flying spot raster size M to be 450 holes. The speed v of the scanning spot 86 will be given by the length of a raster line, which in this case is 0.5 inches, divided by the time required for the flying spot to trace one line. For a standard television scan, the beam traces 525 lines in l/ ofa second. Thus we have =7,860 inches/sec.

vet/m -WTY Thus, a mesh mechanical scanning speed v... of say 78 inches per second, which is 0.01 of the spot-scanning speed, would cause a change of only 0.00005 in the ratio of v,/v,. It can be concluded that the system will be insensitive to variations in the mechanical mesh scan speed and to vibration effects which do not exceed maximum velocities of several hundred inches per second. It can also be concluded that mechanical design considerations alone will limit the speed at which the mesh may be mechanically scanned. The greater this mechanical mesh scan speed is made, the less time is required to complete the mesh inspection operation.

The mesh 56 mechanical scan unit compromises the transparent frame 62 which is displaced in its own plane in a line raster motion to sweep the entire mesh area over the optically active area. The frame 62 carries the mesh 56 and the photosensitive fllm 60 on which the mesh defects are to be mapped. The same type of unit may be used whether the optically active area is that of a flying spot raster produced by a cathode ray tube or that of a laser beam which is processed by a lens We! and pa ia t r.-

Tow types of scanning modes which may be used for the mesh mechanical scan are illustrated in FIGS. 9 and 10. The type of sawtooth scan used in conventional television raster scan has some limitation since the rapid retrace, during which the signals would be suppressed, would unnecessarily tax the mechanical design. The sawtooth scanning mode shown in FIG. 9 is redundant since the area of the mesh 56 is scanned twice. This redundancy is not necessarily bad. The great advantage of this system is that constant sweep speeds may be used both in the line sweep direction and in the frame sweep direction. The line sweep needs only to reverse its direction at the end of each line which is simple to do with a reversing switch operating the drive motor at the end of each line sweep. The frame sweep can be arranged, by means of a mechanically operated switch, to stop at the end of the frame. The next mesh is started at this position by reversing the frame sweep motor and the sweep continues until it is completed, after which the motor is again stopped by a mechanically operated switch and the scanner is in a position to start the next meshinspection cycle.

The stop-scanning mode shown in FIG. 10 is nonredundant but its operation is more complex. The line scan occurs in linear back and forth excursions but the frame scan is now intermittent. At the end of each line, the line scan must stop and the frame scan operate until the frame has moved the width of the flying spot raster, after which the line scan picks up again with reversed direction. The trade off between the two modes is the factor of redundancy versus the extra complexity of an intermittent frame scan operation.

It should be noted that with either sawtooth scan or step scan the area covered by the mechanical scan sweep may overlap the mesh 56 as indicated in FIGS. 9 and 10. The edge of the mesh 56 will then produce a defect signal which will define the exact location of defects within the mesh area of the photographic record of defects. Notches or numbers may be used at the mesh edge for identification and orientation reference. The size of the mechanical mesh scan unit will be defined by the maximum mesh size which needs to be inspected. All smaller mesh sizes can then be measured on the unit. The positions of the electrical scan sweep limiting switches may be adjusted to match the sweep area to the mesh area and thus minimize inspection time by decreasing idling time. V H 7 One mesh scanner unit 200 is shown schematically in FIGS. 11a, b and c. This comprises a double slide system carrying the moving platform 62 to which the mesh 56 and the photographic fllm or paper 60 are attached. Suppose the mesh 56 is square of dimensions L by L and the optically active areas are of square dimensions A by A being separated by L B as illustrated in FIG. 11a. A slow-scan movable frame 201 which slides on fixed rails (not shown) is caused to produce the slow mechanical sweep as indicated in FIG. 11b. The slow-scan amplitude is limited by switches 202 and 204 actuated by the movable frame 201 as shown. A motor 206 and circuitry to the switches 202 and 204 remain fixed in this arrangement and the only thing moving is frame 201 with parts attached to it. The platfonn 62 which carries the mesh 56 and the photographic film 60 are supported on a rapid-scan movable frame 207 which rides on rails (not shown) supported by frame 201. This is shown in FIG. [is which, for simplicity, shows only frame 201 and its attached frame 207. Cables supplying power to motor 208 need to travel only with the slow scan motion. The rapid-scan amplitude is limited by switches 210 and 212 actuated by the frame 207.

The system 10 described above produces a photographic map" of the mesh defects. This map has the same scale as the mesh so it could be laid on top of the mesh to locate the defacts. The map also shows the edges of the mesh if the mechanical scan is allowed to extend beyond the mesh boundaries. Fiducial marks in the form of holes or notches in the edges of the mesh may be used for identification or for orientation purposes.

It is possible to obtain more detailed information on the nature of the defects where this is desired as, for example, for quality control purposes. This can be done by stopping the mechanical scan when a defect appears near the center of the flying spot scan area. The center of the scan area can be electronically magnified at the readout tube by decreasing the size of the focused flying spot scan area. This can be done by using a very short focal length lens in this optical system. if the scanned area is decreased in size to one-tenth, the image projected by the monitor tube 14 will be magnified by 10 diameters and this can be recorded on the photographic film 60, provided that the mechanical scan motion has been stopped. Following this, the scanned raster is restored to its original size by replacing the normal lens and the mechanical scanning is continued. This sequence of operations could be initiated by the defect signal itself or could be done under the control of an operator watching a monitor tube.

Another method of marking defects would be to place a mark on the plastic film in which the mesh is encased for mechanical protection. This method bypasses the photographic recording operation. The marking could be done by using a fine jet of marking fluid which would be pulsed by a defect signal. It would be necessary to avoid interference between the optical effects of the mark and the normal operation of the system.

Since the system described operates both in the space domain (the mesh and its map) and in the time domain (the video electrical signal) it is possible to carry out additional processing operations using the video defect signal. For example, the number of defects in a mesh can be counted by an electrical circuit which counts the number of defect signal pulses. This number could be displayed or printed or both. The distribution of defects could also be analyzed by circuits which would sense the intervals between defect signals and display these data as numbers or curves.

lclaim:

1. An image inspection system for inspecting perforated planar structures in the form of a two-dimensional lattice, said system comprising:

a flying spot scanner tube for optically scanning the structure being inspected with a flying spot at a uniform line scanning rate;

a monitor tube for displaying the-resulting scanning video signal such that only defects in the structure are displayed;

an anticorrelation filtering circuit coupled between the scanner tube and the monitor tube, said circuit filtering the video signals from the scanner tube so as to cancel these signals produced by perfect parts of the structure while allowing signals produced by defects in the structure to be displayed by the monitor tube;

means for mechanically scanning the structure relative to the scanner tube to permit inspection of the entire area of the structure; and

a photosensitive film mounted on said mechanical scanning means in such manner that the defects in the structure displayed on the monitor tube are mapped on the film.

2. A system as set forth in claim 1 wherein:

said scanner tube scans the structure in a sawtooth scanning mode.

3. A system as set forth in claim 1 wherein:

said scanner tube scans the structure in a step-scanning mode.

4. A system as set forth in claim 1 wherein:

said anticorrelation filtering circuit is an ultrasonic delay line.

5. A system as set forth in claim 1 wherein:

said anticorrelation filtering circuit is an electromagnetic delay line.

6. A system as set forth in claim 1 wherein:

said anticorrelation filtering circuit is a terminated coaxial cable.

7. A system as set forth in claim 1 wherein:

said film is a direct imaging film requiring no darkroom processing.

8. A system as set forth in claim 1 wherein:

said mechanical scanning means includes a frame and a movable platform upon which the structure and the film are mounted in the same plane adjacent one another such that as the movable platform is mechanically scanned the film is scanned past the monitor tube and the structure is scanned past the scanner tube.

9. An image inspection system for inspecting a perforated metal mesh, said system comprising:

a flying spot scanner tube for optically scanning the mesh being inspected with a flying spot at a uniform linescanning rate;

one lens adjacent the scanner tube for focusing the scanned raster of the scanning tube;

a monitor tube for displaying the resulting scanning video signal such that only defects in the mesh are displayed;

another lens adjacent the mesh on the side of the mesh opposite that of said one lens for focusing the light from the scanning tube passing through the scanned mesh;

a photomultiplier tube for amplifying the focused light from the scanning means passing through the scanned mesh;

an anticorrelation filtering circuit coupled between the scanner tube and the monitor tube, said circuit filtering the video signals from the scanner tube so as to cancel these signals produced by perfect parts of the mesh while allowing signals produced by defects in the mesh to be displayed by the monitor tube;

means for mechanically scanning the mesh relative to the scanner tube to permit inspection of the entire area of the mesh; and

a photosensitive film mounted on said mechanical scanning means in such manner that the defects in the mesh displayed on the monitor tube are mapped on the film.

10. A system as set forth in claim 9 wherein:

said scanner tube scans the mesh in a sawtooth scanning mode.

11. A system as set forth in claim 9 wherein:

said scanner tube scans the mesh in a step-scanning mode.

12. A system as set forth in claim 9 wherein:

said anticorrelation filtering circuit is an ultrasonic delay line.

13. A system as set forth in claim 9 wherein:

said anticorrelation filtering circuit is an electromagnetic delay line.

14. A system as set forth in claim 9 wherein:

said anticorrelation filtering circuit is a terminated coaxial cable.

15. A system as set forth in claim 9 wherein:

said film is a direct imaging film requiring no darkroom processing.

16. A system as set forth in claim 9 wherein:

said mechanical scanning means includes a frame and a movable platform upon which the mesh and the film are mounted in the same plane adjacent one another such that as the movable platform is mechanically scanned the film is scanned past the monitor tube and the structure is scanned past the scanner tube.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 620,63O Dated November 16, 1971 Inventor(s) Rudolph C, Hergenrother It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the abstract, the penultimate line, change "detect" to defect Column 4, line 26, change "3 1/3" to reaching Column 4, line 73 delete "trailing" Column 6, line 21, change "Tow to W0 Signed and sealed this 13th day of June 1972.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents po'wso USCOMM-DC man-Poo a 5 anvnuunl'r rnmflim DIIllfl: "ll 0-in-3 

1. An image inspection system for inspecting perforated planar structures in the form of a two-dimensional lattice, said system comprising: a flying spot scanner tube for optically scanning the structure being inspected with a flying spot at a uniform line scanning rate; a monitor tube for displaying the resulting scanning video signal such that only defects in the structure are displayed; an anticorrelation filtering circuit coupled between the scanner tube and the monitor tube, said circuit filtering the video signals from the scanner tube so as to cancel these signals produced by perfect parts of the structure while allowing signals produced by defects in the structure to be displayed by the monitor tube; means for mechanically scanning the structure relative to the scanner tube to permit inspection of the entire area of the structure; and a photosensitive film mounted on said mechanical scanning means in such manner that the defects in the structure displayed on the monitor tube are mapped on the film.
 2. A system as set forth in claim 1 wherein: said scanner tube scans the structure in a sawtooth scanning mode.
 3. A system as set forth in claim 1 wherein: said scanner tube scans the structure in a step-scanning mode.
 4. A system as set forth in claim 1 wherein: said anticorrelation filtering circuit is an ultrasonic delay line.
 5. A system as set forth in claim 1 wherein: said anticorrelation filtering circuit is an electromagnetic delay line.
 6. A system as set forth in claim 1 wherein: said anticorrelation filtering circuit is a terminated coaxial cable.
 7. A system as set forth in claim 1 wherein: said film is a direct imaging film requiring no darkroom processing.
 8. A system as set forth in claim 1 wherein: said mechanical scanning means includes a frame and a movable platform upon which the structure and the film are mounted in the same plane adjacent one another such that as the movable platform is mechanically scanned the film is scanned past the monitor tube and the structure is scanned past the scanner tube.
 9. An image inspection system for inspecting a perforated metal mesh, said system comprising: a flying spot scanner tube for optically scanning the mesh being inspected with a flying spot at a uniform line-scanning rate; one lens adjacent the scanner tube for focusing the scanned raster of the scanning tube; a monitor tube for displaying the resulting scanning video signal such that only defects in the mesh are displayed; another lens adjacent the mesh on the side of the mesh opposite that of said one lens for focusing the light from the scanning tube passing through the scanned mesh; a photomultiplier tube for amplifying the focused light from the scanning means passing through the scanned mesh; an anticorrelation filtering circuit coupled between the scanner tube and the monitor tube, said circuit filtering the video signals from the scanner tube so as to cancel these signals produced by perfect parts of the mesh while allowing signals produced by defects in the mesh to be displayed by the monitor tube; means for mechanically scanning the mesh relative to the scanner tube to permit inspection of the entire area of the mesh; and a photosensitive film mounted on said mechanical scanning means in such manner that the defects in the mesh displayed on the monitor tube are mapped on tHe film.
 10. A system as set forth in claim 9 wherein: said scanner tube scans the mesh in a sawtooth scanning mode.
 11. A system as set forth in claim 9 wherein: said scanner tube scans the mesh in a step-scanning mode.
 12. A system as set forth in claim 9 wherein: said anticorrelation filtering circuit is an ultrasonic delay line.
 13. A system as set forth in claim 9 wherein: said anticorrelation filtering circuit is an electromagnetic delay line.
 14. A system as set forth in claim 9 wherein: said anticorrelation filtering circuit is a terminated coaxial cable.
 15. A system as set forth in claim 9 wherein: said film is a direct imaging film requiring no darkroom processing.
 16. A system as set forth in claim 9 wherein: said mechanical scanning means includes a frame and a movable platform upon which the mesh and the film are mounted in the same plane adjacent one another such that as the movable platform is mechanically scanned the film is scanned past the monitor tube and the structure is scanned past the scanner tube. 